The invention relates to a corrosion protection or anti-corrosive coating for surface protection of structural steel components, and to a process for welding a component provided with such a coating.
When carrying out electric spot welding for joining two steel plates protected by a polymeric anti-corrosive coating, the welding process is often impaired by the electrical insulating effect of the polymeric coating. Thus, attempts have been made in the art to develop effective anti-corrosive coatings that will not impair the weldability characteristics of the steel component provided with the coating.
An anti-corrosive coating based on zinc-iron-phosphide fillers is known from U.S. Pat. No. 4,119,763. The zinc content of the fillers is partly replaced by non-metallic inhibitors such as zinc yellow in order to improve the weldability of the thus coated steel surface. However, the metallic conductivity of these coatings interacting with the underlying metal leads to the formation of voltaic cells which promote corrosion. Therefore it is additionally necessary to deposit an electrical insulating layer to suppress this formation of voltaic cells.
German Patent Publication 34 12 234 A1 discloses an anti-corrosive primer of which the filler may contain semiconductive compounds. Although the semiconductive compounds suppress the production of voltaic cells, the material has an insufficient conductivity and thus reduces the weldability.
German Patent Laying-Open Publication 2,202,078 discloses an anti-corrosive coating material, which contains oxidized semiconductive compounds for binding hydrogen that evolves during welding due to the decomposition of the binding agents. Although this prevents the formation of blowholes and of pores, the weldability is severely limited, just as in the cases described above.
In view of the above, it is an object of the invention to provide an anti-corrosive coating for steel components that does not hinder the welding of such coated components, and a process for welding structural steel components that have such an anti-corrosive coating for surface protection. The coatings should be easily weldable along with the steel substrate during the welding process, and after the welding operation the coatings should continue to provide good protection against corrosion while preventing the formation of voltaic cells. The invention further aims to avoid or overcome the disadvantages of the prior art and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved according to the invention in an anti-corrosive coating including at least one coating layer that consists essentially of from 3 wt % to 80 wt % of a binder and from 20 wt % to 97 wt % of a filler, wherein the binder comprises a polymeric material and the filler comprises a filling agent and a semiconducting material. The semiconducting material may comprise one or more elemental semiconductors such as silicon or germanium, or one or more compound semiconductors such as a titanium oxide, molybdenum sulfide, molybdenum selenide, gallium arsenide, indium phosphide, tungsten selenide, tungsten sulfide, zinc oxide and zinc sulfide. The filling agent may comprise one or more of titanium oxide, silicon oxide, calcium oxide, zinc oxide, carbon black, or zirconium oxide. Inhibitors, auxiliary additives, and/or plastifiers may be added to the filler and/or the binder as needed for any particular application. The coating may be a single-layer or multi-layer coating.
The above objects have further been achieved according to the invention in a method of welding a structural steel component coated with an anti-corrosive coating as described above. The welding method involves temporarily applying an external or additional energy to the coating so as to temporarily excite the semiconducting material into a conductive state. Then the actual welding of the structural steel component is carried out using any known welding technique, and particularly electric spot welding, while the semiconducting material is in the conductive state. After the welding has been completed, the semiconducting material returns from the conductive state to a substantially non-conductive state.
According to the invention, the elemental or compound semiconductors are added to the anti-corrosive coating as part of the filler. The semiconductors are only electrically conductive while external energy sufficient to provide the necessary excitation energy is applied to the coating, for example in the form of light or heat. According to the invention this energy is supplied just prior to and/or during the welding process, so that the weldability of such coated sheets is improved. The applied excitation energy is separate from and additional to the energy that is applied for carrying out the welding, particularly because the excitation energy must be applied before the welding can be initiated. Namely, the energized semiconducting material of the coating is brought into a conductive state by the application of the excitation energy just prior to and/or during the welding process per se, so that the welding will not be hampered by insulative effects of the coating.
Preferably the excitation energy, which excites the elemental or compound semiconductors into the state of conductivity, is controlled by the supply of heat during the welding process. In an alternative embodiment, the energy can be applied to the coating in the form of incident radiation or light of an appropriate wavelength corresponding to a photon energy matching the energy band gap of the semiconducting material.
Once the welding process is completed and the external application of an excitation or activation energy is discontinued, the semiconducting materials return to a semiconductive state with a substantial insulating quality. Moreover, when not being welded, the semiconducting materials, and especially silicon, offer corrosion inhibiting characteristics because they can act as an anode for the steel being protected, whereby the steel is protected cathodically from corrosion by the silicon. In addition to this cathodic protection, the semiconducting materials work as corrosion inhibiting oxygen scavengers. As the polymer matrix ages, water and oxygen slowly diffuse through it. The semiconducting materials react with these substances, producing oxides, such as SiO2. Consequently, the concentration of these diffused substances decreases and the corrosion of the substrate is delayed.
The use of silicon as a component of the filler has the particular advantage that high-purity silicon is readily available as a waste product of wafer production in the electronics industry. Cost-effectively obtainable with constant quality, it can be used in the anti-corrosive coating according to the present invention.
With the special coating composition according to the present invention, it is not necessary to use environmentally harmful heavy metals and chromates, as is the case in some prior art anti-corrosive coatings.